Temperature assessment using dielectric properties of an insulated conductor heater with selected electrical insulation

ABSTRACT

A system for assessing one or more temperatures in an opening in a subsurface formation includes an electrical conductor and electrical insulation at least partially surrounding the electrical conductor. The electrical insulation includes magnesium oxide and a ferroelectric material. An electrically conductive sheath at least partially surrounds the electrical insulation. A profile of one or more dielectric properties of the electrical insulation along a length of the electrical insulation is assessed during use to assess a temperature profile with spatial resolution along the length of the electrical insulation.

This non-provisional application claims priority from U.S. ProvisionalApplication Ser. No. 61/808,494 filed Apr. 4, 2013, which is herebyincorporated by reference in its entirety.

BACKGROUND

1. Field of the Invention

The present invention relates generally to methods and systems forproduction of hydrocarbons, hydrogen, and/or other products from varioussubsurface formations such as hydrocarbon containing formations.

2. Description of Related Art

Hydrocarbons obtained from subterranean formations are often used asenergy resources, as feedstocks, and as consumer products. Concerns overdepletion of available hydrocarbon resources and concerns over decliningoverall quality of produced hydrocarbons have led to development ofprocesses for more efficient recovery, processing and/or use ofavailable hydrocarbon resources. In situ processes may be used to removehydrocarbon materials from subterranean formations that were previouslyinaccessible and/or too expensive to extract using available methods.Chemical and/or physical properties of hydrocarbon material in asubterranean formation may need to be changed to allow hydrocarbonmaterial to be more easily removed from the subterranean formationand/or increase the value of the hydrocarbon material. The chemical andphysical changes may include in situ reactions that produce removablefluids, composition changes, solubility changes, density changes, phasechanges, and/or viscosity changes of the hydrocarbon material in theformation.

Large deposits of heavy hydrocarbons (heavy oil and/or tar) contained inrelatively permeable formations (for example in tar sands) are found inNorth America, South America, Africa, and Asia. Tar can be surface-minedand upgraded to lighter hydrocarbons such as crude oil, naphtha,kerosene, and/or gas oil. Surface milling processes may further separatethe bitumen from sand. The separated bitumen may be converted to lighthydrocarbons using conventional refinery methods. Mining and upgradingtar sand is usually substantially more expensive than producing lighterhydrocarbons from conventional oil reservoirs.

Typical temperature measurement methods may be difficult and/orexpensive to implement for use in assessing a temperature profile of aheater located in a subsurface formation for heating in an in situ heattreatment process. The desire is for a temperature profile that includesmultiple temperatures along the length or a portion of the heater in thesubsurface formation. Thermocouples are one possible solution; however,thermocouples provide only one temperature at one location and two wiresare generally needed for each thermocouple. Thus, to obtain atemperature profile along a length of the heater, multiple pairs ofwires are needed. In some cases, the wellbore for the heater may not belarge enough to accommodate the number of wires needed for adequatetemperature monitoring. The risk of failure of one or more of thethermocouples (or their associated wires) is increased with the use ofmultiple wires in the subsurface wellbore. In addition, thermocouplesinstalled in high temperature applications (>300° C.) may suffer from aphenomenon known as temperature measurement drift. Temperaturemeasurement drift may be a significant source of error.

Another possible solution is the use of a fiber optic cable temperaturesensor system. The fiber optic cable system provides a temperatureprofile along a length of the heater. Commercially available fiber opticcable systems, however, typically only have operating temperature rangesup to about 300° C. and are susceptible to mechanical damage because ofthe softening of the fiber and/or fiber coatings resulting in thevarious materials adhering to each other. Thus, these systems are notsuitable for measurement of higher temperatures encountered whileheating the subsurface formation during the in situ heat treatmentprocess. Some experimental fiber optic cable systems are suitable foruse at these higher temperatures but these systems may be too expensivefor implementation in a commercial process (for example, a large fieldof heaters). Thus, there is a need for a simple, inexpensive system thatallows temperature assessment at one or more locations along a length ofthe subsurface heater used in the in situ heat treatment process.

SUMMARY

Embodiments described herein generally relate to systems, methods, andheaters for treating a subsurface formation. Embodiments describedherein also generally relate to heaters that have novel componentstherein. Such heaters can be obtained by using the systems and methodsdescribed herein.

In certain embodiments, the invention provides one or more systems,methods, and/or heaters. In some embodiments, the systems, methods,and/or heaters are used for treating a subsurface formation.

In certain embodiments, a system for assessing a temperature in anopening in a subsurface formation includes: an electrical conductor;electrical insulation at least partially surrounding the electricalconductor, the electrical insulation comprising magnesium oxide and aferroelectric material; and an electrically conductive sheath at leastpartially surrounding the electrical insulation; wherein a profile ofone or more dielectric properties of the electrical insulation along alength of the electrical insulation are assessed during use to assess atemperature profile with spatial resolution along the length of theelectrical insulation.

In certain embodiments, a method for assessing a temperature in anopening in a subsurface formation, includes: assessing a profile of oneor more dielectric properties with spatial resolution along a length ofan insulated conductor located in the opening, wherein the insulatedconductor comprises: an electrical conductor; electrical insulation atleast partially surrounding the electrical conductor, the electricalinsulation comprising magnesium oxide and a ferroelectric material; andan electrically conductive sheath at least partially surrounding theelectrical insulation; and assessing a temperature profile with spatialresolution along the length of the insulated conductor based on theassessed profile of the one or more dielectric properties.

In further embodiments, features from specific embodiments may becombined with features from other embodiments. For example, featuresfrom one embodiment may be combined with features from any of the otherembodiments.

In further embodiments, treating a subsurface formation is performedusing any of the methods, systems, or heaters described herein.

In further embodiments, additional features may be added to the specificembodiments described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the present invention may become apparent to those skilledin the art with the benefit of the following detailed description andupon reference to the accompanying drawings in which:

FIG. 1 shows a schematic view of an embodiment of a portion of an insitu heat treatment system for treating a hydrocarbon containingformation.

FIG. 2 depicts an embodiment of an insulated conductor heat source.

FIG. 3 depicts another embodiment of an insulated conductor heat source.

FIG. 4 depicts another embodiment of an insulated conductor heat source.

FIG. 5 depicts an example of a plot of dielectric constant versustemperature for magnesium oxide insulation in one embodiment of aninsulated conductor heater.

FIG. 6 depicts an example of a plot of loss tangent (tan δ) versustemperature for magnesium oxide insulation in one embodiment of aninsulated conductor heater.

FIG. 7 depicts dielectric constant versus temperature for undopedbismuth titanate at several frequencies.

FIG. 8 depicts dielectric constant versus temperature for bismuthtitanate doped with 0.74% by weight niobium at several frequencies.

FIG. 9 depicts an embodiment of a length of an insulated conductor.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and may herein be described in detail. Thedrawings may not be to scale. It should be understood, however, that thedrawings and detailed description thereto are not intended to limit theinvention to the particular form disclosed, but on the contrary, theintention is to cover all modifications, equivalents and alternativesfalling within the spirit and scope of the present invention as definedby the appended claims.

DETAILED DESCRIPTION

The following description generally relates to systems and methods fortreating hydrocarbons in the formations. Such formations may be treatedto yield hydrocarbon products, hydrogen, and other products.

“Alternating current (AC)” refers to a time-varying current thatreverses direction substantially sinusoidally. AC produces skin effectelectricity flow in a ferromagnetic conductor.

“Annular region” is the region between an outer conduit and an innerconduit positioned in the outer conduit.

“API gravity” refers to API gravity at 15.5° C. (60° F.). API gravity isas determined by ASTM Method D6822 or ASTM Method D1298.

“ASTM” refers to American Standard Testing and Materials.

In the context of reduced heat output heating systems, apparatus, andmethods, the term “automatically” means such systems, apparatus, andmethods function in a certain way without the use of external control(for example, external controllers such as a controller with atemperature sensor and a feedback loop, PID controller, or predictivecontroller).

“Asphalt/bitumen” refers to a semi-solid, viscous material soluble incarbon disulfide. Asphalt/bitumen may be obtained from refiningoperations or produced from subsurface formations.

“Condensable hydrocarbons” are hydrocarbons that condense at 25° C. andone atmosphere absolute pressure. Condensable hydrocarbons may include amixture of hydrocarbons having carbon numbers greater than 4.“Non-condensable hydrocarbons” are hydrocarbons that do not condense at25° C. and one atmosphere absolute pressure. Non-condensablehydrocarbons may include hydrocarbons having carbon numbers less than 5.

“Coupled” means either a direct connection or an indirect connection(for example, one or more intervening connections) between one or moreobjects or components. The phrase “directly connected” means a directconnection between objects or components such that the objects orcomponents are connected directly to each other so that the objects orcomponents operate in a “point of use” manner.

“Curie temperature” is the temperature above which a ferromagneticmaterial loses all of its ferromagnetic properties. In addition tolosing all of its ferromagnetic properties above the Curie temperature,the ferromagnetic material begins to lose its ferromagnetic propertieswhen an increasing electrical current is passed through theferromagnetic material.

A “fluid” may be, but is not limited to, a gas, a liquid, an emulsion, aslurry, and/or a stream of solid particles that has flow characteristicssimilar to liquid flow.

A “formation” includes one or more hydrocarbon containing layers, one ormore non-hydrocarbon layers, an overburden, and/or an underburden.“Hydrocarbon layers” refer to layers in the formation that containhydrocarbons. The hydrocarbon layers may contain non-hydrocarbonmaterial and hydrocarbon material. The “overburden” and/or the“underburden” include one or more different types of impermeablematerials. For example, the overburden and/or underburden may includerock, shale, mudstone, or wet/tight carbonate. In some embodiments of insitu heat treatment processes, the overburden and/or the underburden mayinclude a hydrocarbon containing layer or hydrocarbon containing layersthat are relatively impermeable and are not subjected to temperaturesduring in situ heat treatment processing that result in significantcharacteristic changes of the hydrocarbon containing layers of theoverburden and/or the underburden. For example, the underburden maycontain shale or mudstone, but the underburden is not allowed to heat topyrolysis temperatures during the in situ heat treatment process. Insome cases, the overburden and/or the underburden may be somewhatpermeable.

“Formation fluids” refer to fluids present in a formation and mayinclude pyrolyzation fluid, synthesis gas, mobilized hydrocarbons, andwater (steam). Formation fluids may include hydrocarbon fluids as wellas non-hydrocarbon fluids. The term “mobilized fluid” refers to fluidsin a hydrocarbon containing formation that are able to flow as a resultof thermal treatment of the formation. “Produced fluids” refer to fluidsremoved from the formation.

A “heat source” is any system for providing heat to at least a portionof a formation substantially by conductive and/or radiative heattransfer. For example, a heat source may include electrically conductingmaterials and/or electric heaters such as an insulated conductor, anelongated member, and/or a conductor disposed in a conduit. A heatsource may also include systems that generate heat by burning a fuelexternal to or in a formation. The systems may be surface burners,downhole gas burners, flameless distributed combustors, and naturaldistributed combustors. In some embodiments, heat provided to orgenerated in one or more heat sources may be supplied by other sourcesof energy. The other sources of energy may directly heat a formation, orthe energy may be applied to a transfer medium that directly orindirectly heats the formation. It is to be understood that one or moreheat sources that are applying heat to a formation may use differentsources of energy. Thus, for example, for a given formation some heatsources may supply heat from electrically conducting materials orelectric resistance heaters. Some heat sources may provide heat fromcombustion and some heat sources may provide heat from one or more otherenergy sources (for example, chemical reactions, solar energy, windenergy, biomass, or other sources of renewable energy). A chemicalreaction may include an exothermic reaction (for example, an oxidationreaction). A heat source may also include an electrically conductingmaterial and/or a heater that provides heat to a zone proximate and/orsurrounding a heating location such as a heater well.

A “heater” is any system or heat source for generating heat in a well ora near wellbore region. Heaters may be, but are not limited to, electricheaters, burners, combustors that react with material in or producedfrom a formation, and/or combinations thereof.

“Heavy hydrocarbons” are viscous hydrocarbon fluids. Heavy hydrocarbonsmay include highly viscous hydrocarbon fluids such as heavy oil, tar,and/or asphalt. Heavy hydrocarbons may include carbon and hydrogen, aswell as smaller concentrations of sulfur, oxygen, and nitrogen.Additional elements may also be present in heavy hydrocarbons in traceamounts. Heavy hydrocarbons may be classified by API gravity. Heavyhydrocarbons generally have an API gravity below about 20°. Heavy oil,for example, generally has an API gravity of about 10-20°, whereas targenerally has an API gravity below about 10°. The viscosity of heavyhydrocarbons is generally greater than about 100 centipoise at 15° C.Heavy hydrocarbons may include aromatics or other complex ringhydrocarbons.

Heavy hydrocarbons may be found in a relatively permeable formation. Therelatively permeable formation may include heavy hydrocarbons entrainedin, for example, sand or carbonate. “Relatively permeable” is defined,with respect to formations or portions thereof, as an averagepermeability of 10 millidarcy or more (for example, 10 or 100millidarcy). “Relatively low permeability” is defined, with respect toformations or portions thereof, as an average permeability of less thanabout 10 millidarcy. One darcy is equal to about 0.99 squaremicrometers. An impermeable layer generally has a permeability of lessthan about 0.1 millidarcy.

Certain types of formations that include heavy hydrocarbons may alsoinclude, but are not limited to, natural mineral waxes or naturalasphaltites. “Natural mineral waxes” typically occur in substantiallytubular veins that may be several meters wide, several kilometers long,and hundreds of meters deep. “Natural asphaltites” include solidhydrocarbons of an aromatic composition and typically occur in largeveins. In situ recovery of hydrocarbons from formations such as naturalmineral waxes and natural asphaltites may include melting to form liquidhydrocarbons and/or solution mining of hydrocarbons from the formations.

“Hydrocarbons” are generally defined as molecules formed primarily bycarbon and hydrogen atoms. Hydrocarbons may also include other elementssuch as, but not limited to, halogens, metallic elements, nitrogen,oxygen, and/or sulfur. Hydrocarbons may be, but are not limited to,kerogen, bitumen, pyrobitumen, oils, natural mineral waxes, andasphaltites. Hydrocarbons may be located in or adjacent to mineralmatrices in the earth. Matrices may include, but are not limited to,sedimentary rock, sands, silicilytes, carbonates, diatomites, and otherporous media. “Hydrocarbon fluids” are fluids that include hydrocarbons.Hydrocarbon fluids may include, entrain, or be entrained innon-hydrocarbon fluids such as hydrogen, nitrogen, carbon monoxide,carbon dioxide, hydrogen sulfide, water, and ammonia.

An “in situ conversion process” refers to a process of heating ahydrocarbon containing formation from heat sources to raise thetemperature of at least a portion of the formation above a pyrolysistemperature so that pyrolyzation fluid is produced in the formation.

An “in situ heat treatment process” refers to a process of heating ahydrocarbon containing formation with heat sources to raise thetemperature of at least a portion of the formation above a temperaturethat results in mobilized fluid, visbreaking, and/or pyrolysis ofhydrocarbon containing material so that mobilized fluids, visbrokenfluids, and/or pyrolyzation fluids are produced in the formation.

“Insulated conductor” refers to any elongated material that is able toconduct electricity and that is covered, in whole or in part, by anelectrically insulating material.

“Kerogen” is a solid, insoluble hydrocarbon that has been converted bynatural degradation and that principally contains carbon, hydrogen,nitrogen, oxygen, and sulfur. Coal and oil shale are typical examples ofmaterials that contain kerogen. “Bitumen” is a non-crystalline solid orviscous hydrocarbon material that is substantially soluble in carbondisulfide. “Oil” is a fluid containing a mixture of condensablehydrocarbons.

“Pyrolysis” is the breaking of chemical bonds due to the application ofheat. For example, pyrolysis may include transforming a compound intoone or more other substances by heat alone. Heat may be transferred to asection of the formation to cause pyrolysis.

“Pyrolyzation fluids” or “pyrolysis products” refers to fluid producedsubstantially during pyrolysis of hydrocarbons. Fluid produced bypyrolysis reactions may mix with other fluids in a formation. Themixture would be considered pyrolyzation fluid or pyrolyzation product.As used herein, “pyrolysis zone” refers to a volume of a formation (forexample, a relatively permeable formation such as a tar sands formation)that is reacted or reacting to form a pyrolyzation fluid.

“Superposition of heat” refers to providing heat from two or more heatsources to a selected section of a formation such that the temperatureof the formation at least at one location between the heat sources isinfluenced by the heat sources.

“Tar” is a viscous hydrocarbon that generally has a viscosity greaterthan about 10,000 centipoise at 15° C. The specific gravity of targenerally is greater than 1.000. Tar may have an API gravity less than10°.

A “tar sands formation” is a formation in which hydrocarbons arepredominantly present in the form of heavy hydrocarbons and/or tarentrained in a mineral grain framework or other host lithology (forexample, sand or carbonate). Examples of tar sands formations includeformations such as the Athabasca formation, the Grosmont formation, andthe Peace River formation, all three in Alberta, Canada; and the Fajaformation in the Orinoco belt in Venezuela.

“Temperature limited heater” generally refers to a heater that regulatesheat output (for example, reduces heat output) above a specifiedtemperature without the use of external controls such as temperaturecontrollers, power regulators, rectifiers, or other devices. Temperaturelimited heaters may be AC (alternating current) or modulated (forexample, “chopped”) DC (direct current) powered electrical resistanceheaters.

“Thickness” of a layer refers to the thickness of a cross section of thelayer, wherein the cross section is normal to a face of the layer.

“Time-varying current” refers to electrical current that produces skineffect electricity flow in a ferromagnetic conductor and has a magnitudethat varies with time. Time-varying current includes both alternatingcurrent (AC) and modulated direct current (DC).

A “u-shaped wellbore” refers to a wellbore that extends from a firstopening in the formation, through at least a portion of the formation,and out through a second opening in the formation. In this context, thewellbore may be only roughly in the shape of a “v” or “u”, with theunderstanding that the “legs” of the “u” do not need to be parallel toeach other, or perpendicular to the “bottom” of the “u” for the wellboreto be considered “u-shaped”.

“Upgrade” refers to increasing the quality of hydrocarbons. For example,upgrading heavy hydrocarbons may result in an increase in the APIgravity of the heavy hydrocarbons.

“Viscosity” refers to kinematic viscosity at 40° C. unless otherwisespecified. Viscosity is as determined by ASTM Method D445.

The term “wellbore” refers to a hole in a formation made by drilling orinsertion of a conduit into the formation. A wellbore may have asubstantially circular cross section, or another cross-sectional shape.As used herein, the terms “well” and “opening,” when referring to anopening in the formation may be used interchangeably with the term“wellbore.”

A formation may be treated in various ways to produce many differentproducts. Different stages or processes may be used to treat theformation during an in situ heat treatment process. In some embodiments,one or more sections of the formation are solution mined to removesoluble minerals from the sections. Solution mining minerals may beperformed before, during, and/or after the in situ heat treatmentprocess. In some embodiments, the average temperature of one or moresections being solution mined may be maintained below about 120° C.

In some embodiments, one or more sections of the formation are heated toremove water from the sections and/or to remove methane and othervolatile hydrocarbons from the sections. In some embodiments, theaverage temperature may be raised from ambient temperature totemperatures below about 220° C. during removal of water and volatilehydrocarbons.

In some embodiments, one or more sections of the formation are heated totemperatures that allow for movement and/or visbreaking of hydrocarbonsin the formation. In some embodiments, the average temperature of one ormore sections of the formation are raised to mobilization temperaturesof hydrocarbons in the sections (for example, to temperatures rangingfrom 100° C. to 250° C., from 120° C. to 240° C., or from 150° C. to230° C.).

In some embodiments, one or more sections are heated to temperaturesthat allow for pyrolysis reactions in the formation. In someembodiments, the average temperature of one or more sections of theformation may be raised to pyrolysis temperatures of hydrocarbons in thesections (for example, temperatures ranging from 230° C. to 900° C.,from 240° C. to 400° C. or from 250° C. to 350° C.).

Heating the hydrocarbon containing formation with a plurality of heatsources may establish thermal gradients around the heat sources thatraise the temperature of hydrocarbons in the formation to desiredtemperatures at desired heating rates. The rate of temperature increasethrough the mobilization temperature range and/or the pyrolysistemperature range for desired products may affect the quality andquantity of the formation fluids produced from the hydrocarboncontaining formation. Slowly raising the temperature of the formationthrough the mobilization temperature range and/or pyrolysis temperaturerange may allow for the production of high quality, high API gravityhydrocarbons from the formation. Slowly raising the temperature of theformation through the mobilization temperature range and/or pyrolysistemperature range may allow for the removal of a large amount of thehydrocarbons present in the formation as hydrocarbon product.

In some in situ heat treatment embodiments, a portion of the formationis heated to a desired temperature instead of slowly heating thetemperature through a temperature range. In some embodiments, thedesired temperature is 300° C., 325° C., or 350° C. Other temperaturesmay be selected as the desired temperature.

Superposition of heat from heat sources allows the desired temperatureto be relatively quickly and efficiently established in the formation.Energy input into the formation from the heat sources may be adjusted tomaintain the temperature in the formation substantially at a desiredtemperature.

Mobilization and/or pyrolysis products may be produced from theformation through production wells. In some embodiments, the averagetemperature of one or more sections is raised to mobilizationtemperatures and hydrocarbons are produced from the production wells.The average temperature of one or more of the sections may be raised topyrolysis temperatures after production due to mobilization decreasesbelow a selected value. In some embodiments, the average temperature ofone or more sections may be raised to pyrolysis temperatures withoutsignificant production before reaching pyrolysis temperatures. Formationfluids including pyrolysis products may be produced through theproduction wells.

In some embodiments, the average temperature of one or more sections maybe raised to temperatures sufficient to allow synthesis gas productionafter mobilization and/or pyrolysis. In some embodiments, hydrocarbonsmay be raised to temperatures sufficient to allow synthesis gasproduction without significant production before reaching thetemperatures sufficient to allow synthesis gas production. For example,synthesis gas may be produced in a temperature range from about 400° C.to about 1200° C., about 500° C. to about 1100° C., or about 550° C. toabout 1000° C. A synthesis gas generating fluid (for example, steamand/or water) may be introduced into the sections to generate synthesisgas. Synthesis gas may be produced from production wells.

Solution mining, removal of volatile hydrocarbons and water, mobilizinghydrocarbons, pyrolyzing hydrocarbons, generating synthesis gas, and/orother processes may be performed during the in situ heat treatmentprocess. In some embodiments, some processes may be performed after thein situ heat treatment process. Such processes may include, but are notlimited to, recovering heat from treated sections, storing fluids (forexample, water and/or hydrocarbons) in previously treated sections,and/or sequestering carbon dioxide in previously treated sections.

FIG. 1 depicts a schematic view of an embodiment of a portion of the insitu heat treatment system for treating the hydrocarbon containingformation. The in situ heat treatment system may include barrier wells100. Barrier wells are used to form a barrier around a treatment area.The barrier inhibits fluid flow into and/or out of the treatment area.Barrier wells include, but are not limited to, dewatering wells, vacuumwells, capture wells, injection wells, grout wells, freeze wells, orcombinations thereof. In some embodiments, barrier wells 100 aredewatering wells. Dewatering wells may remove liquid water and/orinhibit liquid water from entering a portion of the formation to beheated, or to the formation being heated. As shown, barrier wells 100extend only along one side of heat sources 102, but the barrier wellstypically encircle all heat sources 102 used, or to be used, to heat atreatment area of the formation.

Heat sources 102 are placed in at least a portion of the formation. Heatsources 102 may include heaters such as insulated conductors,conductor-in-conduit heaters, surface burners, flameless distributedcombustors, and/or natural distributed combustors. Heat sources 102 mayalso include other types of heaters. Heat sources 102 provide heat to atleast a portion of the formation to heat hydrocarbons in the formation.Energy may be supplied to heat sources 102 through supply lines 104.Supply lines 104 may be structurally different depending on the type ofheat source or heat sources used to heat the formation. Supply lines 104for heat sources may transmit electricity for electric heaters, maytransport fuel for combustors, or may transport heat exchange fluid thatis circulated in the formation. In some embodiments, electricity for anin situ heat treatment process may be provided by a nuclear power plantor nuclear power plants. The use of nuclear power may allow forreduction or elimination of carbon dioxide emissions from the in situheat treatment process.

When the formation is heated, the heat input into the formation maycause expansion of the formation and geomechanical motion. The heatsources may be turned on before, at the same time, or during adewatering process. Computer simulations may model formation response toheating. The computer simulations may be used to develop a pattern andtime sequence for activating heat sources in the formation so thatgeomechanical motion of the formation does not adversely affect thefunctionality of heat sources, production wells, and other equipment inthe formation.

Heating the formation may cause an increase in permeability and/orporosity of the formation. Increases in permeability and/or porosity mayresult from a reduction of mass in the formation due to vaporization andremoval of water, removal of hydrocarbons, and/or creation of fractures.Fluid may flow more easily in the heated portion of the formationbecause of the increased permeability and/or porosity of the formation.Fluid in the heated portion of the formation may move a considerabledistance through the formation because of the increased permeabilityand/or porosity. The considerable distance may be over 1000 m dependingon various factors, such as permeability of the formation, properties ofthe fluid, temperature of the formation, and pressure gradient allowingmovement of the fluid. The ability of fluid to travel considerabledistance in the formation allows production wells 106 to be spacedrelatively far apart in the formation.

Production wells 106 are used to remove formation fluid from theformation. In some embodiments, production well 106 includes a heatsource. The heat source in the production well may heat one or moreportions of the formation at or near the production well. In some insitu heat treatment process embodiments, the amount of heat supplied tothe formation from the production well per meter of the production wellis less than the amount of heat applied to the formation from a heatsource that heats the formation per meter of the heat source. Heatapplied to the formation from the production well may increase formationpermeability adjacent to the production well by vaporizing and removingliquid phase fluid adjacent to the production well and/or by increasingthe permeability of the formation adjacent to the production well byformation of macro and/or micro fractures.

More than one heat source may be positioned in the production well. Aheat source in a lower portion of the production well may be turned offwhen superposition of heat from adjacent heat sources heats theformation sufficiently to counteract benefits provided by heating theformation with the production well. In some embodiments, the heat sourcein an upper portion of the production well may remain on after the heatsource in the lower portion of the production well is deactivated. Theheat source in the upper portion of the well may inhibit condensationand reflux of formation fluid.

In some embodiments, the heat source in production well 106 allows forvapor phase removal of formation fluids from the formation. Providingheating at or through the production well may: (1) inhibit condensationand/or refluxing of production fluid when such production fluid ismoving in the production well proximate the overburden, (2) increaseheat input into the formation, (3) increase production rate from theproduction well as compared to a production well without a heat source,(4) inhibit condensation of high carbon number compounds (C₆hydrocarbons and above) in the production well, and/or (5) increaseformation permeability at or proximate the production well.

Subsurface pressure in the formation may correspond to the fluidpressure generated in the formation. As temperatures in the heatedportion of the formation increase, the pressure in the heated portionmay increase as a result of thermal expansion of in situ fluids,increased fluid generation and vaporization of water. Controlling rateof the fluid removal from the formation may allow for control ofpressure in the formation. Pressure in the formation may be determinedat a number of different locations, such as near or at production wells,near or at heat sources, and near or at monitor wells.

In some hydrocarbon containing formations, production of hydrocarbonsfrom the formation is inhibited until at least some hydrocarbons in theformation have been mobilized and/or pyrolyzed. Formation fluid may beproduced from the formation when the formation fluid is of a selectedquality. In some embodiments, the selected quality includes an APIgravity of at least about 20°, 30°, or 40°. Inhibiting production untilat least some hydrocarbons are mobilized and/or pyrolyzed may increaseconversion of heavy hydrocarbons to light hydrocarbons. Inhibitinginitial production may minimize the production of heavy hydrocarbonsfrom the formation. Production of substantial amounts of heavyhydrocarbons may require expensive equipment and/or reduce the life ofproduction equipment.

In some hydrocarbon containing formations, hydrocarbons in the formationmay be heated to mobilization and/or pyrolysis temperatures beforesubstantial permeability has been generated in the heated portion of theformation. An initial lack of permeability may inhibit the transport ofgenerated fluids to production wells 106. During initial heating, fluidpressure in the formation may increase proximate heat sources 102. Theincreased fluid pressure may be released, monitored, altered, and/orcontrolled through one or more heat sources 102. For example, selectedheat sources 102 or separate pressure relief wells may include pressurerelief valves that allow for removal of some fluid from the formation.

In some embodiments, pressure generated by expansion of mobilizedfluids, pyrolysis fluids or other fluids generated in the formation maybe allowed to increase although an open path to production wells 106 orany other pressure sink may not yet exist in the formation. The fluidpressure may be allowed to increase towards a lithostatic pressure.Fractures in the hydrocarbon containing formation may form when thefluid approaches the lithostatic pressure. For example, fractures mayform from heat sources 102 to production wells 106 in the heated portionof the formation. The generation of fractures in the heated portion mayrelieve some of the pressure in the portion. Pressure in the formationmay have to be maintained below a selected pressure to inhibit unwantedproduction, fracturing of the overburden or underburden, and/or cokingof hydrocarbons in the formation.

After mobilization and/or pyrolysis temperatures are reached andproduction from the formation is allowed, pressure in the formation maybe varied to alter and/or control a composition of produced formationfluid, to control a percentage of condensable fluid as compared tonon-condensable fluid in the formation fluid, and/or to control an APIgravity of formation fluid being produced. For example, decreasingpressure may result in production of a larger condensable fluidcomponent. The condensable fluid component may contain a largerpercentage of olefins.

In some in situ heat treatment process embodiments, pressure in theformation may be maintained high enough to promote production offormation fluid with an API gravity of greater than 20°. Maintainingincreased pressure in the formation may inhibit formation subsidenceduring in situ heat treatment. Maintaining increased pressure may reduceor eliminate the need to compress formation fluids at the surface totransport the fluids in collection conduits to treatment facilities.

Maintaining increased pressure in a heated portion of the formation maysurprisingly allow for production of large quantities of hydrocarbons ofincreased quality and of relatively low molecular weight. Pressure maybe maintained so that formation fluid produced has a minimal amount ofcompounds above a selected carbon number. The selected carbon number maybe at most 25, at most 20, at most 12, or at most 8. Some high carbonnumber compounds may be entrained in vapor in the formation and may beremoved from the formation with the vapor. Maintaining increasedpressure in the formation may inhibit entrainment of high carbon numbercompounds and/or multi-ring hydrocarbon compounds in the vapor. Highcarbon number compounds and/or multi-ring hydrocarbon compounds mayremain in a liquid phase in the formation for significant time periods.The significant time periods may provide sufficient time for thecompounds to pyrolyze to form lower carbon number compounds.

Generation of relatively low molecular weight hydrocarbons is believedto be due, in part, to autogenous generation and reaction of hydrogen ina portion of the hydrocarbon containing formation. For example,maintaining an increased pressure may force hydrogen generated duringpyrolysis into the liquid phase within the formation. Heating theportion to a temperature in a pyrolysis temperature range may pyrolyzehydrocarbons in the formation to generate liquid phase pyrolyzationfluids. The generated liquid phase pyrolyzation fluids components mayinclude double bonds and/or radicals. Hydrogen (H₂) in the liquid phasemay reduce double bonds of the generated pyrolyzation fluids, therebyreducing a potential for polymerization or formation of long chaincompounds from the generated pyrolyzation fluids. In addition, H₂ mayalso neutralize radicals in the generated pyrolyzation fluids. H₂ in theliquid phase may inhibit the generated pyrolyzation fluids from reactingwith each other and/or with other compounds in the formation.

Formation fluid produced from production wells 106 may be transportedthrough collection piping 108 to treatment facilities 110. Formationfluids may also be produced from heat sources 102. For example, fluidmay be produced from heat sources 102 to control pressure in theformation adjacent to the heat sources. Fluid produced from heat sources102 may be transported through tubing or piping to collection piping 108or the produced fluid may be transported through tubing or pipingdirectly to treatment facilities 110. Treatment facilities 110 mayinclude separation units, reaction units, upgrading units, fuel cells,turbines, storage vessels, and/or other systems and units for processingproduced formation fluids. The treatment facilities may formtransportation fuel from at least a portion of the hydrocarbons producedfrom the formation. In some embodiments, the transportation fuel may bejet fuel, such as JP-8.

An insulated conductor may be used as an electric heater element of aheater or a heat source. The insulated conductor may include an innerelectrical conductor (core) surrounded by an electrical insulator and anouter electrical conductor (jacket). The electrical insulator mayinclude mineral insulation (for example, magnesium oxide) or otherelectrical insulation.

In certain embodiments, the insulated conductor is placed in an openingin a hydrocarbon containing formation. In some embodiments, theinsulated conductor is placed in an uncased opening in the hydrocarboncontaining formation. Placing the insulated conductor in an uncasedopening in the hydrocarbon containing formation may allow heat transferfrom the insulated conductor to the formation by radiation as well asconduction. Using an uncased opening may facilitate retrieval of theinsulated conductor from the well, if necessary.

In some embodiments, an insulated conductor is placed within a casing inthe formation; may be cemented within the formation; or may be packed inan opening with sand, gravel, or other fill material. The insulatedconductor may be supported on a support member positioned within theopening. The support member may be a cable, rod, or a conduit (forexample, a pipe). The support member may be made of a metal, ceramic,inorganic material, or combinations thereof. Because portions of asupport member may be exposed to formation fluids and heat during use,the support member may be chemically resistant and/or thermallyresistant.

Ties, spot welds, and/or other types of connectors may be used to couplethe insulated conductor to the support member at various locations alonga length of the insulated conductor. The support member may be attachedto a wellhead at an upper surface of the formation. In some embodiments,the insulated conductor has sufficient structural strength such that asupport member is not needed. The insulated conductor may, in manyinstances, have at least some flexibility to inhibit thermal expansiondamage when undergoing temperature changes.

In certain embodiments, insulated conductors are placed in wellboreswithout support members and/or centralizers. An insulated conductorwithout support members and/or centralizers may have a suitablecombination of temperature and corrosion resistance, creep strength,length, thickness (diameter), and metallurgy that will inhibit failureof the insulated conductor during use.

FIG. 2 depicts a perspective view of an end portion of an embodiment ofinsulated conductor 112. Insulated conductor 112 may have any desiredcross-sectional shape such as, but not limited to, round (depicted inFIG. 2), triangular, ellipsoidal, rectangular, hexagonal, or irregular.In certain embodiments, insulated conductor 112 includes core 114,electrical insulator 116, and jacket 118. Core 114 may resistively heatwhen an electrical current passes through the core. Alternating ortime-varying current and/or direct current may be used to provide powerto core 114 such that the core resistively heats.

In some embodiments, electrical insulator 116 inhibits current leakageand arcing to jacket 118. Electrical insulator 116 may thermally conductheat generated in core 114 to jacket 118. Jacket 118 may radiate orconduct heat to the formation. In certain embodiments, insulatedconductor 112 is 1000 m or more in length. Longer or shorter insulatedconductors may also be used to meet specific application needs. Thedimensions of core 114, electrical insulator 116, and jacket 118 ofinsulated conductor 112 may be selected such that the insulatedconductor has enough strength to be self supporting even at upperworking temperature limits. Such insulated conductors may be suspendedfrom wellheads or supports positioned near an interface between anoverburden and a hydrocarbon containing formation without the need forsupport members extending into the hydrocarbon containing formationalong with the insulated conductors.

Insulated conductor 112 may be designed to operate at power levels of upto about 1650 watts/meter or higher. In certain embodiments, insulatedconductor 112 operates at a power level between about 500 watts/meterand about 1150 watts/meter when heating a formation. Insulated conductor112 may be designed so that a maximum voltage level at a typicaloperating temperature does not cause substantial thermal and/orelectrical breakdown of electrical insulator 116. Insulated conductor112 may be designed such that jacket 118 does not exceed a temperaturethat will result in a significant reduction in corrosion resistanceproperties of the jacket material. In certain embodiments, insulatedconductor 112 may be designed to reach temperatures within a rangebetween about 650° C. and about 900° C. Insulated conductors havingother operating ranges may be formed to meet specific operationalrequirements.

FIG. 2 depicts insulated conductor 112 having a single core 114. In someembodiments, insulated conductor 112 has two or more cores 114. Forexample, a single insulated conductor may have three cores. Core 114 maybe made of metal or another electrically conductive material. Thematerial used to form core 114 may include, but not be limited to,nichrome, copper, nickel, carbon steel, stainless steel, andcombinations thereof. In certain embodiments, core 114 is chosen to havea diameter and a resistivity at operating temperatures such that itsresistance, as derived from Ohm's law, makes it electrically andstructurally stable for the chosen power dissipation per meter, thelength of the heater, and/or the maximum voltage allowed for the corematerial.

In some embodiments, core 114 is made of different materials along alength of insulated conductor 112. For example, a first section of core114 may be made of a material that has a significantly lower resistancethan a second section of the core. The first section may be placedadjacent to a formation layer that does not need to be heated to as higha temperature as a second formation layer that is adjacent to the secondsection. The resistivity of various sections of core 114 may be adjustedby having a variable diameter and/or by having core sections made ofdifferent materials.

Electrical insulator 116 may be made of a variety of materials. Commonlyused powders may include, but are not limited to, MgO, Al₂O₃, Zirconia,BeO, different chemical variations of Spinels, and combinations thereof.MgO may provide good thermal conductivity and electrical insulationproperties. The desired electrical insulation properties include lowleakage current and high dielectric strength. A low leakage currentdecreases the possibility of thermal breakdown and the high dielectricstrength decreases the possibility of arcing across the insulator.Thermal breakdown can occur if the leakage current causes a progressiverise in the temperature of the insulator leading also to arcing acrossthe insulator.

Jacket 118 may be an outer metallic layer or electrically conductivelayer. Jacket 118 may be in contact with hot formation fluids. Jacket118 may be made of material having a high resistance to corrosion atelevated temperatures. Alloys that may be used in a desired operatingtemperature range of jacket 118 include, but are not limited to, 304stainless steel, 310 stainless steel, Incoloy® 800, and Inconel® 600(Inco Alloys International, Huntington, W. Va., U.S.A.). The thicknessof jacket 118 may have to be sufficient to last for three to ten yearsin a hot and corrosive environment. A thickness of jacket 118 maygenerally vary between about 1 mm and about 2.5 mm. For example, a 1.3mm thick, 310 stainless steel outer layer may be used as jacket 118 toprovide good chemical resistance to sulfidation corrosion in a heatedzone of a formation for a period of over 3 years. Larger or smallerjacket thicknesses may be used to meet specific applicationrequirements.

One or more insulated conductors may be placed within an opening in aformation to form a heat source or heat sources. Electrical current maybe passed through each insulated conductor in the opening to heat theformation. Alternately, electrical current may be passed throughselected insulated conductors in an opening. The unused conductors maybe used as backup heaters. Insulated conductors may be electricallycoupled to a power source in any convenient manner. Each end of aninsulated conductor may be coupled to lead-in cables that pass through awellhead. Such a configuration typically has a 180° bend (a “hairpin”bend) or turn located near a bottom of the heat source. An insulatedconductor that includes a 180° bend or turn may not require a bottomtermination, but the 180° bend or turn may be an electrical and/orstructural weakness in the heater. Insulated conductors may beelectrically coupled together in series, in parallel, or in series andparallel combinations. In some embodiments of heat sources, electricalcurrent may pass into the conductor of an insulated conductor and may bereturned through the jacket of the insulated conductor by connectingcore 114 to jacket 118 (shown in FIG. 2) at the bottom of the heatsource.

In some embodiments, three insulated conductors 112 are electricallycoupled in a 3-phase wye configuration to a power supply. FIG. 3 depictsan embodiment of three insulated conductors in an opening in asubsurface formation coupled in a wye configuration. FIG. 4 depicts anembodiment of three insulated conductors 112 that are removable fromopening 120 in the formation. No bottom connection may be required forthree insulated conductors in a wye configuration. Alternately, allthree insulated conductors of the wye configuration may be connectedtogether near the bottom of the opening. The connection may be madedirectly at ends of heating sections of the insulated conductors or atends of cold pins (less resistive sections) coupled to the heatingsections at the bottom of the insulated conductors. The bottomconnections may be made with insulator filled and sealed canisters orwith epoxy filled canisters. The insulator may be the same compositionas the insulator used as the electrical insulation.

Three insulated conductors 112 depicted in FIGS. 3 and 4 may be coupledto support member 122 using centralizers 124. Alternatively, insulatedconductors 112 may be strapped directly to support member 122 usingmetal straps. Centralizers 124 may maintain a location and/or inhibitmovement of insulated conductors 112 on support member 122. Centralizers124 may be made of metal, ceramic, or combinations thereof. The metalmay be stainless steel or any other type of metal able to withstand acorrosive and high temperature environment. In some embodiments,centralizers 124 are bowed metal strips welded to the support member atdistances less than about 6 m. A ceramic used in centralizer 124 may be,but is not limited to, Al₂O₃, MgO, or another electrical insulator.Centralizers 124 may maintain a location of insulated conductors 112 onsupport member 122 such that movement of insulated conductors isinhibited at operating temperatures of the insulated conductors.Insulated conductors 112 may also be somewhat flexible to withstandexpansion of support member 122 during heating.

Support member 122, insulated conductor 112, and centralizers 124 may beplaced in opening 120 in hydrocarbon layer 126. Insulated conductors 112may be coupled to bottom conductor junction 128 using cold pin 130.Bottom conductor junction 128 may electrically couple each insulatedconductor 112 to each other. Bottom conductor junction 128 may includematerials that are electrically conducting and do not melt attemperatures found in opening 120. Cold pin 130 may be an insulatedconductor having lower electrical resistance than insulated conductor112.

Lead-in conductor 132 may be coupled to wellhead 134 to provideelectrical power to insulated conductor 112. Lead-in conductor 132 maybe made of a relatively low electrical resistance conductor such thatrelatively little heat is generated from electrical current passingthrough the lead-in conductor. In some embodiments, the lead-inconductor is a rubber or polymer insulated stranded copper wire. In someembodiments, the lead-in conductor is a mineral insulated conductor witha copper core. Lead-in conductor 132 may couple to wellhead 134 atsurface 136 through a sealing flange located between overburden 138 andsurface 136. The sealing flange may inhibit fluid from escaping fromopening 120 to surface 136.

In certain embodiments, lead-in conductor 132 is coupled to insulatedconductor 112 using transition conductor 140. Transition conductor 140may be a less resistive portion of insulated conductor 112. Transitionconductor 140 may be referred to as “cold pin” of insulated conductor112. Transition conductor 140 may be designed to dissipate aboutone-tenth to about one-fifth of the power per unit length as isdissipated in a unit length of the primary heating section of insulatedconductor 112. Transition conductor 140 may typically be between about1.5 m and about 15 m, although shorter or longer lengths may be used toaccommodate specific application needs. In an embodiment, the conductorof transition conductor 140 is copper. The electrical insulator oftransition conductor 140 may be the same type of electrical insulatorused in the primary heating section. A jacket of transition conductor140 may be made of corrosion resistant material.

In certain embodiments, transition conductor 140 is coupled to lead-inconductor 132 by a splice or other coupling joint. Splices may also beused to couple transition conductor 140 to insulated conductor 112.Splices may have to withstand a temperature equal to half of a targetzone operating temperature or, in some cases, up to about 85% of thetarget zone operating temperature. Density of electrical insulation inthe splice should in many instances be high enough to withstand therequired temperature and the operating voltage.

In some embodiments, as shown in FIG. 3, packing material 142 is placedbetween overburden casing 144 and opening 120. In some embodiments,reinforcing material 146 may secure overburden casing 144 to overburden138. Packing material 142 may inhibit fluid from flowing from opening120 to surface 136. Reinforcing material 146 may include, for example,Class G or Class H Portland cement mixed with silica flour for improvedhigh temperature performance, slag or silica flour, and/or a mixturethereof. In some embodiments, reinforcing material 146 extends radiallya width of from about 5 cm to about 25 cm.

As shown in FIGS. 3 and 4, support member 122 and lead-in conductor 132may be coupled to wellhead 134 at surface 136 of the formation. Surfaceconductor 148 may enclose reinforcing material 146 and couple towellhead 134. Embodiments of surface conductors may extend to depths ofapproximately 3 m to approximately 515 m into an opening in theformation. Alternatively, the surface conductor may extend to a depth ofapproximately 9 m into the formation. Electrical current may be suppliedfrom a power source to insulated conductor 112 to generate heat due tothe electrical resistance of the insulated conductor. Heat generatedfrom three insulated conductors 112 may transfer within opening 120 toheat at least a portion of hydrocarbon layer 126.

Heat generated by insulated conductors 112 may heat at least a portionof a hydrocarbon containing formation. In some embodiments, heat istransferred to the formation substantially by radiation of the generatedheat to the formation. Some heat may be transferred by conduction orconvection of heat due to gases present in the opening. The opening maybe an uncased opening, as shown in FIGS. 3 and 4. An uncased openingeliminates cost associated with thermally cementing the heater to theformation, costs associated with a casing, and/or costs of packing aheater within an opening. In addition, heat transfer by radiation istypically more efficient than by conduction, so the heaters may beoperated at lower temperatures in an open wellbore. Conductive heattransfer during initial operation of a heat source may be enhanced bythe addition of a gas in the opening. The gas may be maintained at apressure up to about 27 bars absolute. The gas may include, but is notlimited to, carbon dioxide and/or helium. An insulated conductor heaterin an open wellbore may advantageously be free to expand or contract toaccommodate thermal expansion and contraction. An insulated conductorheater may advantageously be removable or redeployable from an openwellbore.

In certain embodiments, an insulated conductor heater assembly isinstalled or removed using a spooling assembly. More than one spoolingassembly may be used to install both the insulated conductor and asupport member simultaneously. Alternatively, the support member may beinstalled using a coiled tubing unit. The heaters may be un-spooled andconnected to the support as the support is inserted into the well. Theelectric heater and the support member may be un-spooled from thespooling assemblies. Spacers may be coupled to the support member andthe heater along a length of the support member. Additional spoolingassemblies may be used for additional electric heater elements.

Temperature limited heaters may be in configurations and/or may includematerials that provide automatic temperature limiting properties for theheater at certain temperatures. Examples of temperature limited heatersmay be found in U.S. Pat. No. 6,688,387 to Wellington et al.; U.S. Pat.No. 6,991,036 to Sumnu-Dindoruk et al.; U.S. Pat. No. 6,698,515 toKaranikas et al.; U.S. Pat. No. 6,880,633 to Wellington et al.; U.S.Pat. No. 6,782,947 to de Rouffignac et al.; U.S. Pat. No. 6,991,045 toVinegar et al.; U.S. Pat. No. 7,073,578 to Vinegar et al.; U.S. Pat. No.7,121,342 to Vinegar et al.; U.S. Pat. No. 7,320,364 to Fairbanks; U.S.Pat. No. 7,527,094 to McKinzie et al.; U.S. Pat. No. 7,584,789 to Mo etal.; U.S. Pat. No. 7,533,719 to Hinson et al.; and U.S. Pat. No.7,562,707 to Miller; U.S. Patent Application Publication Nos.2009-0071652 to Vinegar et al.; 2009-0189617 to Burns et al.;2010-0071903 to Prince-Wright et al.; and 2010-0096137 to Nguyen et al.,each of which is incorporated by reference as if fully set forth herein.Temperature limited heaters are dimensioned to operate with ACfrequencies (for example, 60 Hz AC) or with modulated DC current.

In certain embodiments, ferromagnetic materials are used in temperaturelimited heaters. Ferromagnetic material may self-limit temperature at ornear the Curie temperature of the material and/or the phasetransformation temperature range to provide a reduced amount of heatwhen a time-varying current is applied to the material. In certainembodiments, the ferromagnetic material self-limits temperature of thetemperature limited heater at a selected temperature that isapproximately the Curie temperature and/or in the phase transformationtemperature range. In certain embodiments, the selected temperature iswithin about 35° C., within about 25° C., within about 20° C., or withinabout 10° C. of the Curie temperature and/or the phase transformationtemperature range. In certain embodiments, ferromagnetic materials arecoupled with other materials (for example, highly conductive materials,high strength materials, corrosion resistant materials, or combinationsthereof) to provide various electrical and/or mechanical properties.Some parts of the temperature limited heater may have a lower resistance(caused by different geometries and/or by using different ferromagneticand/or non-ferromagnetic materials) than other parts of the temperaturelimited heater. Having parts of the temperature limited heater withvarious materials and/or dimensions allows for tailoring the desiredheat output from each part of the heater.

Temperature limited heaters may be more reliable than other heaters.Temperature limited heaters may be less apt to break down or fail due tohot spots in the formation. In some embodiments, temperature limitedheaters allow for substantially uniform heating of the formation. Insome embodiments, temperature limited heaters are able to heat theformation more efficiently by operating at a higher average heat outputalong the entire length of the heater. The temperature limited heateroperates at the higher average heat output along the entire length ofthe heater because power to the heater does not have to be reduced tothe entire heater, as is the case with typical constant wattage heaters,if a temperature along any point of the heater exceeds, or is about toexceed, a maximum operating temperature of the heater. Heat output fromportions of a temperature limited heater approaching a Curie temperatureand/or the phase transformation temperature range of the heaterautomatically reduces without controlled adjustment of the time-varyingcurrent applied to the heater. The heat output automatically reduces dueto changes in electrical properties (for example, electrical resistance)of portions of the temperature limited heater. Thus, more power issupplied by the temperature limited heater during a greater portion of aheating process.

In certain embodiments, the system including temperature limited heatersinitially provides a first heat output and then provides a reduced(second heat output) heat output, near, at, or above the Curietemperature and/or the phase transformation temperature range of anelectrically resistive portion of the heater when the temperaturelimited heater is energized by a time-varying current. The first heatoutput is the heat output at temperatures below which the temperaturelimited heater begins to self-limit. In some embodiments, the first heatoutput is the heat output at a temperature about 50° C., about 75° C.,about 100° C., or about 125° C. below the Curie temperature and/or thephase transformation temperature range of the ferromagnetic material inthe temperature limited heater.

The temperature limited heater may be energized by time-varying current(alternating current or modulated direct current) supplied at thewellhead. The wellhead may include a power source and other components(for example, modulation components, transformers, and/or capacitors)used in supplying power to the temperature limited heater. Thetemperature limited heater may be one of many heaters used to heat aportion of the formation.

In certain embodiments, the temperature limited heater includes aconductor that operates as a skin effect or proximity effect heater whentime-varying current is applied to the conductor. The skin effect limitsthe depth of current penetration into the interior of the conductor. Forferromagnetic materials, the skin effect is dominated by the magneticpermeability of the conductor. The relative magnetic permeability offerromagnetic materials is typically between 10 and 1000 (for example,the relative magnetic permeability of ferromagnetic materials istypically at least 10 and may be at least 50, 100, 500, 1000 orgreater). As the temperature of the ferromagnetic material is raisedabove the Curie temperature, or the phase transformation temperaturerange, and/or as the applied electrical current is increased, themagnetic permeability of the ferromagnetic material decreasessubstantially and the skin depth expands rapidly (for example, the skindepth expands as the inverse square root of the magnetic permeability).The reduction in magnetic permeability results in a decrease in the ACor modulated DC resistance of the conductor near, at, or above the Curietemperature, the phase transformation temperature range, and/or as theapplied electrical current is increased. When the temperature limitedheater is powered by a substantially constant current source, portionsof the heater that approach, reach, or are above the Curie temperatureand/or the phase transformation temperature range may have reduced heatdissipation. Sections of the temperature limited heater that are not ator near the Curie temperature and/or the phase transformationtemperature range may be dominated by skin effect heating that allowsthe heater to have high heat dissipation due to a higher resistive load.

An advantage of using the temperature limited heater to heathydrocarbons in the formation is that the conductor is chosen to have aCurie temperature and/or a phase transformation temperature range in adesired range of temperature operation. Operation within the desiredoperating temperature range allows substantial heat injection into theformation while maintaining the temperature of the temperature limitedheater, and other equipment, below design limit temperatures. Designlimit temperatures are temperatures at which properties such ascorrosion, creep, and/or deformation are adversely affected. Thetemperature limiting properties of the temperature limited heaterinhibit overheating or burnout of the heater adjacent to low thermalconductivity “hot spots” in the formation. In some embodiments, thetemperature limited heater is able to lower or control heat outputand/or withstand heat at temperatures above 25° C., 37° C., 100° C.,250° C., 500° C., 700° C., 800° C., 900° C., or higher up to 1131° C.,depending on the materials used in the heater.

The temperature limited heater allows for more heat injection into theformation than constant wattage heaters because the energy input intothe temperature limited heater does not have to be limited toaccommodate low thermal conductivity regions adjacent to the heater. Forexample, in Green River oil shale there is a difference of at least afactor of 3 in the thermal conductivity of the lowest richness oil shalelayers and the highest richness oil shale layers. When heating such aformation, substantially more heat is transferred to the formation withthe temperature limited heater than with the conventional heater that islimited by the temperature at low thermal conductivity layers. The heatoutput along the entire length of the conventional heater needs toaccommodate the low thermal conductivity layers so that the heater doesnot overheat at the low thermal conductivity layers and burn out. Theheat output adjacent to the low thermal conductivity layers that are athigh temperature will reduce for the temperature limited heater, but theremaining portions of the temperature limited heater that are not athigh temperature will still provide high heat output. Because heatersfor heating hydrocarbon formations typically have long lengths (forexample, at least 10 m, 100 m, 300 m, 500 m, 1 km or more up to about 10km), the majority of the length of the temperature limited heater may beoperating below the Curie temperature and/or the phase transformationtemperature range while only a few portions are at or near the Curietemperature and/or the phase transformation temperature range of thetemperature limited heater.

The use of temperature limited heaters allows for efficient transfer ofheat to the formation. Efficient transfer of heat allows for reductionin time needed to heat the formation to a desired temperature. Forexample, in Green River oil shale, pyrolysis typically requires 9.5years to 10 years of heating when using a 12 m heater well spacing withconventional constant wattage heaters. For the same heater spacing,temperature limited heaters may allow a larger average heat output whilemaintaining heater equipment temperatures below equipment design limittemperatures. Pyrolysis in the formation may occur at an earlier timewith the larger average heat output provided by temperature limitedheaters than the lower average heat output provided by constant wattageheaters. For example, in Green River oil shale, pyrolysis may occur in 5years using temperature limited heaters with a 12 m heater well spacing.Temperature limited heaters counteract hot spots due to inaccurate wellspacing or drilling where heater wells come too close together. Incertain embodiments, temperature limited heaters allow for increasedpower output over time for heater wells that have been spaced too farapart, or limit power output for heater wells that are spaced too closetogether. Temperature limited heaters also supply more power in regionsadjacent to the overburden and underburden to compensate for temperaturelosses in these regions.

Temperature limited heaters may be advantageously used in many types offormations. For example, in tar sands formations or relatively permeableformations containing heavy hydrocarbons, temperature limited heatersmay be used to provide a controllable low temperature output forreducing the viscosity of fluids, mobilizing fluids, and/or enhancingthe radial flow of fluids at or near the wellbore or in the formation.Temperature limited heaters may be used to inhibit excess coke formationdue to overheating of the near wellbore region of the formation.

In some embodiments, the use of temperature limited heaters eliminatesor reduces the need for expensive temperature control circuitry. Forexample, the use of temperature limited heaters eliminates or reducesthe need to perform temperature logging and/or the need to use fixedthermocouples on the heaters to monitor potential overheating at hotspots.

The temperature limited heaters may be used in conductor-in-conduitheaters. In some embodiments of conductor-in-conduit heaters, themajority of the resistive heat is generated in the conductor, and theheat radiatively, conductively and/or convectively transfers to theconduit. In some embodiments of conductor-in-conduit heaters, themajority of the resistive heat is generated in the conduit.

In some embodiments, a relatively thin conductive layer is used toprovide the majority of the electrically resistive heat output of thetemperature limited heater at temperatures up to a temperature at ornear the Curie temperature and/or the phase transformation temperaturerange of the ferromagnetic conductor. Such a temperature limited heatermay be used as the heating member in an insulated conductor heater. Theheating member of the insulated conductor heater may be located inside asheath with an insulation layer between the sheath and the heatingmember.

Current techniques allow for the measurement of dielectric properties ofinsulation along a length of the insulation (measurement of dielectricproperties distributed along the length of the insulation). Thesetechniques provide a profile of the dielectric properties with a spatialresolution (space between measurements) based on the type of insulationand the abilities of the measurement system. These techniques arecurrently used to assess dielectric properties and detect insulationflaws and/or insulation damage. Examples of current techniques are axialtomography and line resonance analysis. A version of axial tomography(Mashikian Axial Tomography) is provided by Instrument ManufacturingCompany (IMCORP) (Storrs, Conn., U.S.A.). Mashikian Axial Tomography isdisclosed in U.S. Pat. Application Pub. No. 2008-0048668 to Mashikian. Aversion of line resonance analysis (LIRA) is provided by Wirescan AS(Halden, Norway). Wirescan AS LIRA is disclosed in International Pat.Pub. No. WO 2007/040406 to Fantoni.

The assessment of dielectric properties (using either the currenttechniques or modified versions of these techniques) may be used incombination with information about the temperature dependence ofdielectric properties to assess a temperature profile of one or moreenergized heaters (heaters that are powered and providing heat). Thetemperature dependence data of the dielectric properties may be foundfrom simulation and/or experimentation. Examples of dielectricproperties of the insulation that may be assessed over time include, butare not limited to, dielectric constant and loss tangent. FIG. 5 depictsan example of a plot of dielectric constant versus temperature formagnesium oxide insulation in one embodiment of an insulated conductorheater. FIG. 6 depicts an example of a plot of loss tangent (tan δ),measured at 60 Hz, versus temperature for magnesium oxide insulation inone embodiment of an insulated conductor heater.

It should be noted that the temperature dependent behavior of adielectric property may vary based on certain factors. Factors that mayaffect the temperature dependent behavior of the dielectric propertyinclude, but are not limited to, the type of insulation, the dimensionsof the insulation, the time the insulation is exposed to environment(for example, heat from the heater), the composition (chemistry) of theinsulation, moisture content, and the compaction of the insulation.Thus, it is typically necessary to measure (either by simulation and/orexperimentation) the temperature dependent behavior of the dielectricproperty for the embodiment of insulation that is to be used in aselected heater.

In certain embodiments, one or more dielectric properties of theinsulation in a heater having electrical insulation are assessed(measured) and compared to temperature dependence data of the dielectricproperties to assess (determine) a temperature profile along a length ofthe heater (for example, the entire length of the heater or a portion ofthe heater). For example, the temperature of an insulated conductorheater (such as a mineral insulated (MI) cable heater) may be assessedbased on dielectric properties of the insulation used in the heater.Since the temperature dependence of the dielectric property measured isknown or estimated from simulation and/or experimentation, the measureddielectric property at a location along the heater may be used to assessthe temperature of the heater at that location. Using techniques thatmeasure the dielectric properties at multiple locations along a lengthof the heater (as is possible with current techniques), a temperatureprofile along that heater length may be provided.

In some embodiments, as shown by the plots in FIGS. 5 and 6, thedielectric properties are more sensitive to temperature at highertemperatures (for example, above about 900° F. (482° C.), as shown inFIGS. 5 and 6). Thus, in some embodiments, the temperature of a portionof the insulated conductor heater is assessed by measurement of thedielectric properties at temperatures above about 400° C. (about 760°F.). For example, the temperature of the portion may be assessed bymeasurement of the dielectric properties at temperatures ranging fromabout 400° C., about 450° C., or about 500° C. to about 800° C., about850° C., or about 900° C. These ranges of temperatures are abovetemperatures that can be measured using commercially available fiberoptic cable systems. A fiber optic cable system suitable for use in thehigher temperature ranges may, however, provide measurements with higherspatial resolution than temperature assessment by measurement of thedielectric properties. Thus, in some embodiments, the fiber optic cablesystem operable in the higher temperature ranges may be used tocalibrate temperature assessment by measurement of dielectricproperties.

At temperatures below these temperature ranges (for example, below about400° C.), temperature assessment by measurement of the dielectricproperties may be less accurate. Temperature assessment by measurementof the dielectric properties may, however, provide a reasonable estimateor “average” temperature of portions of the heater. The averagetemperature assessment may be used to assess whether the heater isoperating at temperatures below about 500° C., below about 450° C., orbelow about 400° C.

Temperature assessment by measurement of dielectric properties mayprovide a temperature profile along a length or portion of the insulatedconductor heater (temperature measurements distributed along the lengthor portion of the heater). Measuring the temperature profile is moreuseful for monitoring and controlling the heater as compared to takingtemperature measurements at only selected locations (such as temperaturemeasurement with thermocouples). Multiple thermocouples may be used toprovide a temperature profile. Multiple wires (two per thermocouple),however, would be needed. Temperature assessment by measurement ofdielectric properties uses only one wire for measurement of thetemperature profile, which is simpler and less expensive than usingmultiple thermocouples. In some embodiments, one or more thermocouplesplaced at selected locations are used to calibrate temperatureassessment by measurement of dielectric properties.

In certain embodiments, the dielectric properties of the insulation inan insulated conductor heater are assessed (measured) over a period oftime to assess the temperature and operating characteristics of theheater over the period of time. For example, the dielectric propertiesmay be assessed continuously (or substantially continuously) to providereal-time monitoring of the dielectric properties and the temperature.Monitoring of the dielectric properties and the temperature may be usedto assess the condition of the heater during operation of the heater.For example, comparison of the assessed properties at specific locationsversus the average properties over the length of the heater may provideinformation on the location of hot spots or defects in the heater.

In certain embodiments, an assessment (scan) of the insulated conductorheater before the heater is used to provide heat (for example, abaseline assessment (scan)) provides information on locations of defectsand/or other discontinuities (for example, splices) that causeabnormalities in the dielectric properties of the heater. The baselineassessment may be used to later detect the presence of hot spots ordefects that occur after the heater is energized. For example,assessment of the heater after energizing the heater may show newabnormalities in the dielectric properties of the heater that were notpresent in the baseline assessment. In some embodiments, the baselineassessment is used to assess the quality of insulation in the heater.For example, the baseline assessment may be used to compare the qualityof sections of insulation (for example, blocks of insulation) along thelength of the heater.

In some embodiments, the dielectric properties of the insulation changeover time. For example, the dielectric properties may change over timebecause of changes in the oxygen concentration in the insulation overtime and/or changes in the water content in the insulation over time.Oxygen in the insulation may be consumed by chromium or other metalsused in the insulated conductor heater. Thus, the oxygen concentrationdecreases with time in the insulation and affects the dielectricproperties of the insulation.

Variations in water (moisture) content in the insulation may also affectthe dielectric properties of the insulation. Experimentation and/orsimulation may be used to provide correlation data between dielectricproperties of the insulation and water content in the insulation. Thus,in certain embodiments, assessment of the dielectric properties is usedto detect the water content in the heater during heating of theformation. For example, a decrease in the dielectric value of theinsulation relative to an initial dielectric value of the insulationhaving a known water content indicates water has permeated theinsulation.

In some embodiments, assessment of water content along the heater isused to inhibit overheating of the heater at locations assessed to haveno water content. If the heater is treated as though there is wateralong the entire length of the heater, portions of the heater(especially for relatively long heaters) that do not have water mayoverheat because of the lack of water and high energy input into theheater. Thus, assessment of water content using the dielectricproperties may provide locations (or at least approximate locations) oflittle or no water (non-detectable) content along the length of theheater and the power provided to the heater can be adjusted to inhibitoverheating such locations on the heater.

In certain embodiments, water content along the heater is related towater content in the formation (for example, if the heater is in an openwellbore, water in the formation may contact the heater in the openwellbore). Thus, assessment of water content along the heater usingassessment of the dielectric properties may be used to monitor thepresence of water in the formation surrounding the heater. For example,assessment of water content using dielectric properties may provideinformation on water inflow into the heater wellbore. Inflow of watermay cause the temperature of the heater to decrease until water beginsto boil off (for example, evaporate), at which time, the heatertemperature will begin increasing.

In certain embodiments, assessment of the dielectric properties allowsassessment of when water boils off in the wellbore surrounding theheater. Thus, assessment of the dielectric properties may be used incommissioning (startup) of the heater to detect when water surroundingthe heater is evaporated. During startup of the heater in a waterenvironment, the heater may be at high power to boil off the water. Ifthe power is not turned down when the surrounding environment changesfrom predominantly liquid water to predominantly steam, the heater mayoverheat due to reduced thermal conductivity. Monitoring the dielectricproperties to assess when the water is boiled off allows the heaterpower to be turned down in time to inhibit overheating of the heater.Water boiling off may be detected as a hot spot or defect (for example,a rapid change in temperature) using the assessment of the dielectricproperties. Thus, the heater may be turned down when the hot spot ordefect is detected using the assessment of the dielectric properties toavoid overheating of the heater during startup of the heater.

In some embodiments, assessment of dielectric properties to assess watercontent is used in flow assurance operations (for example, flowassurance operations for offshore heating). The assessment of watercontent using the dielectric properties in the heater allows fordetection of leaks or other failures that may allow water into theheater or pipeline with the heater while the heater is still in positionoffshore. Assessing water inflow while the heater is in positionoffshore reduces the need to remove the heater for assurance onoperation of the heater.

Changes in dielectric properties of the heater over time may be measuredand compensated for through experimental and/or simulated data. Forexample, the insulated conductor heater to be used for temperatureassessment may be heated in an oven or other apparatus and the changesin dielectric properties can be measured over time at varioustemperatures and/or at constant temperatures. In addition, thermocouplesmay be used to calibrate the assessment of dielectric properties changesover time by comparison of thermocouple data to temperature assessed bythe dielectric properties.

In certain embodiments, temperature assessment by measurement ofdielectric properties is performed using a computational system such asa workstation or computer. The computational system may receivemeasurements (assessments) of the dielectric properties along the heaterand correlate these measured dielectric properties to assesstemperatures at one or more locations on the heater. For example, thecomputational system may store data about the relationship of thedielectric properties to temperature (such as the data depicted in FIGS.5 and 6) and/or time, and use this stored data to calculate thetemperatures on the heater based on the measured dielectric properties.In some embodiments, the computational system is used to subtract outfeedback from power (remove the affect of power on the measurement)provided to the heater when the heater is energized.

In certain embodiments, the computational system is used to monitor thedielectric properties of the heater over a period of time and reportand/or control the heater while monitoring the dielectric properties.For example, the computational system may be used to provide an alarm orother report to a user when the dielectric properties (or temperature)vary by more than a selected percentage. In some embodiments, thecomputational system (automatically) turns down the power to the heaterwhen the dielectric properties (or temperature) vary by more than adesired amount. For example, the power may be ramped down by about 33%,about 50%, or more when the temperature of the heater varies above thedesired temperature by more than a desired amount. In some embodiments,the power is turned down while notifying the user with an alarm or otherreport.

In certain embodiments, temperature assessment by dielectric propertiesmeasurement is performed on an energized heater providing heat to thesubsurface formation (for example, at least a portion of an insulatedconductor heater provided with electric power to resistively heat andproviding at least some heat to the subsurface formation from theportion of the insulated conductor). Assessing temperature on theenergized heater allows for detection of defects in the insulation onthe device actually providing heat to the formation. Assessingtemperature on the energized heater, however, may be more difficult dueto attenuation of signal along the heater because the heater isresistively heating. This attenuation may inhibit seeing further alongthe length of the heater (deeper into the formation along the heater).In some embodiments, temperatures in the upper sections of heaters(sections of the heater closer to the overburden, for example, the upperhalf or upper third of the heater) may be more important for assessmentbecause these sections have higher voltages applied to the heater, areat higher temperatures, and are at higher risk for failure or generationof hot spots. The signal attenuation in the temperature assessment bydielectric properties measurement may not be as significant a factor inthese upper sections because of the proximity of these sections to thesurface.

In some embodiments, power to the insulated conductor heater is turnedoff before performing the temperature assessment. Power is then returnedto the insulated conductor heater after the temperature assessment.Thus, the insulated conductor heater is subjected to a heating on/offcycle to assess temperature. This on/off cycle may, however, reduce thelifetime of the heater due to the thermal cycling. In addition, theheater may cool off during the non-energized time period and provideless accurate temperature information (less accurate information on theactual working temperature of the heater).

In certain embodiments, temperature assessment by dielectric propertiesmeasurement is performed on an insulated conductor that is not to beused for heating or not configured for heating. Such an insulatedconductor may be a separate insulated conductor temperature probe. Insome embodiments, the insulated conductor temperature probe is anon-energized heater (for example, an insulated conductor heater notpowered). The insulated conductor temperature probe may be a stand-alonedevice that can be located in an opening in the subsurface formation tomeasure temperature in the opening. In some embodiments, the insulatedconductor temperature probe is a looped probe that goes out and backinto the opening with signals transmitted in one direction on the probe.In some embodiments, the insulated conductor temperature probe is asingle, hanging probe with the signal transmitted along the core andreturned along the sheath of the insulated conductor.

In certain embodiments, the insulated conductor temperature probeincludes a copper core (to provide better conductance to the end of thecable and better spatial resolution) surrounded by magnesium oxideinsulation and an outer metal sheath. The outer metal sheath may be madeof any material suitable for use in the subsurface opening. For example,the outer metal sheath may be a stainless steel sheath or an innersheath of copper wrapped with an outer sheath of stainless steel.Typically, the insulated conductor temperature probe operates up totemperatures and pressures that can be withstood by the outer metalsheath.

In some embodiments, the insulated conductor temperature probe islocated adjacent to or near an energized heater in the opening tomeasure temperatures along the energized heater. There may be atemperature difference between the insulated conductor temperature probeand the energized heater (for example, between about 50° C. and 100° C.temperature differences). This temperature difference may be assessedthrough experimentation and/or simulation and accounted for in thetemperature measurements. The temperature difference may also becalibrated using one or more thermocouples attached to the energizedheater.

In some embodiments, one or more thermocouples are attached to theinsulated conductor used for temperature assessment (either an energizedinsulated conductor heater or a non-energized insulated conductortemperature probe). The attached thermocouples may be used forcalibration and/or backup measurement of the temperature assessed on theinsulated conductor by dielectric property measurement. In someembodiments, calibration and/or backup temperature indication isachieved by assessment of the resistance variation of the core of theinsulated conductor at a given applied voltage. Temperature may beassessed by knowing the resistance versus temperature profile of thecore material at the given voltage. In some embodiments, the insulatedconductor is a loop and current induced in the loop from energizedheaters in the subsurface opening provides input for the resistancemeasurement.

In certain embodiments, insulation material properties in the insulatedconductor are varied to provide different sensitivities to temperaturefor the insulated conductor. Examples of insulation material propertiesthat may be varied include, but are not limited to, the chemical andphase composition, the microstructure, and/or the mixture of insulatingmaterials. Varying the insulation material properties in the insulatedconductor allows the insulated conductor to be tuned to a selectedtemperature range and/or to increase the sensitivity in assessment oftemperature using the insulated conductor. The selected temperaturerange may be selected, for example, for a desired application of theinsulated conductor.

Insulated conductors (for example, mineral insulated conductors) usedfor in situ heat treatment processes in hydrocarbon containingformations typically use magnesium oxide as the electrical insulator.Although assessing temperatures using dielectric properties of insulatedconductors with magnesium oxide insulation may provide some desiredinformation, the sensitivity to temperature for the dielectric constantof magnesium oxide may not be sufficient for reliable detection oftemperature at desired heater temperatures used during the in situ heattreatment process. For example, as shown in FIG. 5, the dielectricconstant of magnesium oxide increases from about 8 at about 800° F.(about 427° C.) to about 16 at about 1600° F. (about 871° C.), and thusthe dielectric constant increases only about 0.018 per every degreeCelsius increase in temperature. These small changes in the dielectricconstant in the desired temperature ranges for magnesium oxide may makeit difficult to detect temperature differences along a length of theinsulated conductor.

In certain embodiments, ferroelectric materials are added to theelectrical insulator in the insulated conductor to increase thesensitivity for temperature assessment using dielectric properties ofthe electrical insulator. Adding the ferroelectric material may causethe dielectric constant of the electrical insulator to change morerapidly with temperature and increase the sensitivity in measuring thedielectric constant versus temperature. Examples of ferroelectricmaterials that may be added to the electrical insulator include, but arenot limited to, bismuth titanate and doped bismuth titanate (forexample, niobium doped bismuth titanate). In some embodiments, it may bepossible to use other materials that exhibit similar properties asferroelectric materials such as any material having a high dielectricconstant with the dielectric constant varying strongly with temperaturein a desired temperature range.

In certain embodiments, a selected amount of the ferroelectric material(for example, bismuth titanate) is added to magnesium oxide to form theelectrical insulator for the insulated conductor. For example, theselected amount of the ferroelectric material may be added to electricalinsulator 116, shown in FIG. 2. In certain embodiments, the amount offerroelectric material in the electrical insulator is small relative tothe amount of magnesium oxide in the electrical insulator. In certainembodiments, the electrical insulator includes at most about 5% byweight ferroelectric material (with the remainder being substantiallymagnesium oxide). In other embodiments, the electrical insulatorincludes at most about 10% by weight ferroelectric material, at mostabout 7.5% by weight ferroelectric material, or at most 2.5% by weightferroelectric material with the remainder being substantially magnesiumoxide.

Magnesium oxide is typically used as the electrical insulator ininsulated conductors for subsurface applications as magnesium oxide hasgood breakdown voltage values and low electrical conductivity to avoidelectrical shorting between the core and the sheath in the subsurface.The amount of ferroelectric material added to magnesium oxide may bekept small to maintain desired electrical properties in the electricalinsulator while increasing the dielectric constant and increasing thesensitivity to temperature of the dielectric constant for the insulatedconductor. The amount of ferroelectric material may be kept at or belowthe selected amount to, for example, keep the electrical conductivitylow enough to maintain the attenuation of the signal used to determinethe distribution of dielectric constant at an acceptable level. As notedabove, keeping the breakdown voltage above the desired value isimportant in subsurface heating applications to avoid electrical shortsat the high voltages used for long insulated conductors (for example,lengths greater than 10 m or greater than 100 m).

FIG. 7 depicts dielectric constant versus temperature for undopedbismuth titanate at several frequencies. Bismuth titanate is aferroelectric ceramic material that has a higher dielectric constant(which ranges between about 200 and about 500000, as shown in FIG. 7)than magnesium oxide (which ranges between about 7 and 17, as shown inFIG. 5). In addition, the dielectric constant of bismuth titanate ismore sensitive to temperature than the dielectric constant of magnesiumoxide. For example, as shown in FIG. 7, at the frequency of 100 kHz, thedielectric constant increases from about 500 at about 400° C. to about1000 at about 700° C., and thus the dielectric constant increases about1.67 per every degree Celsius increase in the temperature.

The sensitivity to temperature for bismuth titanate at 100 kHz is about92 times the sensitivity of the dielectric constant of magnesium oxidein a similar temperature range (shown in FIG. 5). Thus, addingferroelectric material (such as bismuth titanate) to magnesium oxide inthe electrical insulator (for example, electrical insulator 116, shownin FIG. 2) may increase the sensitivity to temperature of the dielectricconstant in the electrical insulator. Increasing the sensitivity totemperature of the dielectric constant increases the ability to assesstemperature in the insulated conductor using dielectric properties ofthe insulated conductor. Adding even only a small amount offerroelectric material (for example, at most about 5% by weight asdescribed above) may improve the sensitivity to temperature of thedielectric constant in the electrical insulator to levels that allowreliable assessment of the temperature in the insulated conductor usingthe dielectric properties of the electrical insulator.

In certain embodiments, the electrical insulator with ferroelectricmaterial added to magnesium oxide has a dielectric constant thatincreases by a factor of at least about 1.5 for every degree Celsiusincrease in temperature in a desired temperature range. The desiredtemperature range may be, for example, at or near an operatingtemperature range of a heater used to heat a subsurface formation suchas a hydrocarbon containing formation. In some embodiments, the desiredtemperature range is above about 300° C., about 400° C., or about 500°C. up to about 800° C., about 900° C., or about 1000° C. In someembodiments, the electrical insulator with ferroelectric material addedto magnesium oxide has a dielectric constant that increases by a factorof about at least about 2, at least about 5, or at least about 10 forevery degree Celsius increase in temperature in a desired temperaturerange.

As shown in FIG. 7, lowering the frequency applied to bismuth titanatemay further increase the sensitivity to temperature of the dielectricconstant. For example, the dielectric constant of bismuth titanate ishigher and has more sensitivity to temperature at 1 kHz versus 100 kHzand is higher and more sensitive at 20 Hz versus 1 kHz. Thus, in someembodiments, higher sensitivities are achieved by adjusting thefrequency applied to the electrical insulator containing ferroelectricmaterial and magnesium oxide.

In some embodiments, the ferroelectric material is doped with anadditional material to adjust the temperature dependency of thedielectric constant. For example, bismuth titanate may be doped withniobium (Nb) or a similar material. FIG. 8 depicts dielectric constantversus temperature for bismuth titanate doped with 0.74% by weightniobium at several frequencies. As shown in FIG. 8, doping bismuthtitanate with niobium moves the onset of increased temperaturesensitivity to higher temperatures. For example, at 20 Hz, thedielectric constant of undoped bismuth titanate shows high temperaturesensitivity starting at about 100° C. (as shown in FIG. 7) while thedielectric constant of niobium doped bismuth titanate shows hightemperature sensitivity starting at about 400° C. (as shown in FIG. 8).

Shifting the onset of high temperature sensitivity to highertemperatures also increases the sensitivity to temperature of thedielectric constant in the high temperature sensitivity range. Thus, incertain embodiments, doping the ferroelectric material with theadditional material may be used to increase the sensitivity totemperature of the dielectric constant in the desired temperature range.In some embodiments, doping the ferroelectric material with theadditional material may decrease electrical conductivity in theelectrical insulator and/or increase dielectric breakdown voltage in theelectrical insulator.

As shown in FIGS. 7 and 8, in some embodiments, the dielectric constantof bismuth titanate (and thus the electrical insulator) may increase toa selected temperature and then decrease slightly above the selectedtemperature (for example, about 700° C. in FIGS. 7 and 8). In certainembodiments, a computational system such as a workstation or computer isused to perform temperature assessment by measurement of dielectricproperties. As described above, the computational system may receivemeasurements (assessments) of the dielectric properties along the heaterand correlate these measured dielectric properties to assesstemperatures at one or more locations on the heater. Such acomputational system may be used to assess the trend in the profile ofthe dielectric constant with temperature (for example, whether thedielectric constant is increasing or decreasing). The computation systemmay be used to assess whether the dielectric constant being measured isat either the lower temperature (below the selected temperature) or thehigher temperature (above the selected temperature) based on theassessed trend in the dielectric constant profile.

In certain embodiments, the electrical insulator with ferroelectricmaterial added to magnesium oxide is used in the separate insulatedconductor temperature probe described above. The insulated conductortemperature probe may include a copper core and a copper sheath. Theinsulated conductor temperature probe may operate at lower voltage thanan energized insulated conductor. Operating at lower voltage may providelower attenuation in the signal in the insulated conductor temperatureprobe and thus provide higher sensitivity for measurement and morereliable measurements of the dielectric properties of the electricalinsulator. Using an insulated conductor temperature probe withferroelectric material added to magnesium oxide in the electricalinsulator may provide reliable temperature measurement along a length ofa wellbore in which the probe is placed at desired operatingtemperatures for in situ heat treatment processes. For example, theprobe may be placed in a heater wellbore with an energized heater alsoin the wellbore to assess temperatures along the length of the heater.

In certain embodiments, insulation material properties are adjusted tochange the capacitance of the insulation in the heater. FIG. 6 depictsan example of loss tangent versus temperature for magnesium oxideinsulation in an insulated conductor heater. Loss tangent (tan δ) is thereal capacitance divided by the imaginary capacitance. Thus, the curvein FIG. 6 is a capacitance curve for the insulation in the insulatedconductor heater.

The takeoff temperature of the curve (the temperature at which the curvebegins to go vertical or increases dramatically—for example, about 1400°F. (about 760° C.) in the example of FIG. 6) is dependent on thecapacitance of the insulation. Thus, in certain embodiments, thecapacitance of the insulation of the heater is adjusted to change thetakeoff temperature of the capacitance curve. For example, thecapacitance curve, as depicted in FIG. 6, may be shifted left or rightto change the takeoff temperature. If the change in takeoff temperatureof a portion insulation of a heater is known, monitoring of the losstangent allows assessment of when a takeoff temperature of the heater isbeing reached.

In some embodiments, insulation material properties are varied along thelength of the insulated conductor (the insulation material propertiesare different at selected points within the insulated conductor).Varying properties of the insulation material at known locations alongthe length of the insulated conductor allows the measurement of thedielectric properties to give location information and/or provide forself-calibration of the insulated conductor in addition to providingtemperature assessment.

In certain embodiments, the heater has insulation with different knowntakeoff temperatures (different capacitances) at different knownlocations along the length of the heater. FIG. 9 depicts an embodimentof a length of insulated conductor 112. Insulated conductor 112 mayinclude, for example, a plurality of insulated conductor sections112A-112G. Sections 112A-112G may each have different capacitances(takeoff temperatures) with the locations of the transitions (forexample, the locations of the changes in capacitance or takeofftemperature) between the sections being known. In some embodiments, thecapacitance of the insulation is varied by providing insulating blockswith different capacitance along the length of the heater. For example,sections 112A-112G may each have blocks with different capacitances.Knowing the locations of the capacitance changes (for example, thejunctions or borders between sections 112A-112G) and their associatedtakeoff temperatures allows monitoring of the loss tangent of individualsections to provide information on when certain portions of the heaterreach certain temperatures. In some embodiments, the capacitance of theinsulation is tailored to provide temperature assessment of sections ofthe heater using the different takeoff temperatures. For example, alength of a heater may have several sections (for example, sections112A-112D) with takeoff temperatures that incrementally step between twotemperatures, and the loss tangent may be monitored to assess thetemperature of the length based on which section's loss tangent is atits respective takeoff temperature.

In some embodiments, capacitance is measured as a function of lengthalong the heater. The capacitance versus length data may be used toprovide temperature information for the heater, when the heater isenergized, based on the takeoff of the loss tangent.

In some embodiments, the insulated conductor includes a portion withinsulation material properties that allow the portion to act as areflector. The reflector portion may be used to limit temperatureassessment to specific portions of the insulated conductor (for example,a specific length of insulated conductor). One or more reflectorportions may be used to provide spatial markers along the length of theinsulated conductor. For example, reflector portions may be provided atthe transition (junctions) between sections 112A-112G.

Varying the insulation material properties adjusts the activation energyof the insulation material. Typically, increasing the activation energyof the insulation material reduces attenuation in the insulationmaterial and provides better spatial resolution. Lowering the activationenergy typically provides better temperature sensitivity. The activationenergy may be raised or lowered, for example, by adding differentcomponents to the insulation material. For example, adding certaincomponents to magnesium oxide insulation will lower the activationenergy. Examples of components that may be added to magnesium oxide tolower the activation energy include, but are not limited to, titaniumoxide, nickel oxide, and iron oxide.

In some embodiments, temperature is assessed using two or more insulatedconductors. The insulation material in the insulated conductors may havedifferent activation energies to provide a variation in spatialresolution and temperature sensitivity to more accurately assesstemperature in the subsurface opening. The higher activation energyinsulated conductor may be used to provide better spatial resolution andidentify the location of hot spots or other temperature variations moreaccurately while the lower activation energy insulated conductor may beused to provide more accurate temperature measurement at thoselocations.

It is to be understood the invention is not limited to particularsystems described which may, of course, vary. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting. As used in this specification, the singular forms “a”, “an”and “the” include plural referents unless the content clearly indicatesotherwise. Thus, for example, reference to “a layer” includes acombination of two or more layers and reference to “a fluid” includesmixtures of fluids.

In this patent, certain U.S. patents and U.S. patent applications havebeen incorporated by reference. The text of such U.S. patents and U.S.patent applications is, however, only incorporated by reference to theextent that no conflict exists between such text and the otherstatements and drawings set forth herein. In the event of such conflict,then any such conflicting text in such incorporated by reference U.S.patents and U.S. patent applications is specifically not incorporated byreference in this patent.

Further modifications and alternative embodiments of various aspects ofthe invention will be apparent to those skilled in the art in view ofthis description. Accordingly, this description is to be construed asillustrative only and is for the purpose of teaching those skilled inthe art the general manner of carrying out the invention. It is to beunderstood that the forms of the invention shown and described hereinare to be taken as the presently preferred embodiments. Elements andmaterials may be substituted for those illustrated and described herein,parts and processes may be reversed, and certain features of theinvention may be utilized independently, all as would be apparent to oneskilled in the art after having the benefit of this description of theinvention. Changes may be made in the elements described herein withoutdeparting from the spirit and scope of the invention as described in thefollowing claims.

What is claimed is:
 1. A system for assessing a temperature in an opening in a subsurface formation, comprising: an electrical conductor; electrical insulation at least partially surrounding the electrical conductor, the electrical insulation comprising magnesium oxide and a ferroelectric material; and an electrically conductive sheath at least partially surrounding the electrical insulation; wherein a profile of one or more dielectric properties of the electrical insulation along a length of the electrical insulation is assessed during use to assess a temperature profile with spatial resolution along the length of the electrical insulation.
 2. The system of claim 1, wherein the electrical insulator comprises at most about 5% by weight of the ferroelectric material.
 3. The system of claim 1, wherein the ferroelectric material comprises bismuth titanate.
 4. The system of claim 1, wherein the ferroelectric material comprises bismuth titanate doped with niobium.
 5. The system of claim 1, wherein at least one of the dielectric properties comprises a dielectric constant.
 6. The system of claim 1, wherein the electrical insulation comprises a dielectric constant that increases by a factor of at least about 1.5 for every degree Celsius increase in temperature in a desired temperature range.
 7. The system of claim 1, wherein the electrical conductor and the electrical insulation comprise a length of at least about 10 m.
 8. The system of claim 1, wherein the system is located in a wellbore in a subsurface formation.
 9. The system of claim 1, wherein the system is located in a wellbore in a subsurface formation and at least one heater for providing heat to the subsurface formation is located in the wellbore.
 10. The system of claim 1, wherein the electrical conductor and the electrically conductive sheath are used to assess the dielectric properties of the electrical insulation during use.
 11. A method for assessing a temperature in an opening in a subsurface formation, comprising: assessing a profile of one or more dielectric properties with spatial resolution along a length of an insulated conductor located in the opening, wherein the insulated conductor comprises: an electrical conductor; electrical insulation at least partially surrounding the electrical conductor, the electrical insulation comprising magnesium oxide and a ferroelectric material; and an electrically conductive sheath at least partially surrounding the electrical insulation; and assessing a temperature profile with spatial resolution along the length of the insulated conductor based on the assessed profile of the one or more dielectric properties.
 12. The method of claim 11, wherein the electrical insulator comprises at most about 5% by weight of the ferroelectric material.
 13. The method of claim 11, wherein the ferroelectric material comprises bismuth titanate.
 14. The method of claim 11, wherein the ferroelectric material comprises bismuth titanate doped with niobium.
 15. The method of claim 11, wherein at least one of the dielectric properties comprises a dielectric constant.
 16. The method of claim 11, wherein the electrical insulation comprises a dielectric constant that increases by a factor of at least about 1.5 for every degree Celsius increase in temperature in a desired temperature range.
 17. The method of claim 11, further comprising providing heat to the subsurface formation with a heater located in a wellbore in the formation, wherein the insulated conductor is located in the wellbore proximate the heater.
 18. The method of claim 11, further comprising providing electrical power to at least a portion of the insulated conductor, and providing at least some heat to the subsurface formation from the portion of the insulated conductor.
 19. The method of claim 11, further comprising using the electrical conductor and the electrically conductive sheath to assess the dielectric properties of the electrical insulation.
 20. The method of claim 11, further comprising monitoring a trend in the profile of at least one dielectric property to assess a difference between increase in temperature and decrease in temperature for a range of values of the dielectric property.
 21. The method of claim 11, wherein assessing the temperature profile comprises comparing temperature dependence data of the dielectric properties to the assessed profile of the one or more dielectric properties. 